The present invention relates to vinyl aromatic based polymers, including polymer blends, such as polystyrene, high impact polystyrene, styrene acrylonitrile, ABS and other rubber modified styrene copolymers. Such vinyl aromatic based polymers are noted in the industry for wide variations in melt flow rate. Melt flow rate analysis of such polymers yields wide variations in both repeatability and reproducibility in melt flow rate determinations.
When injection molding machines are being operated at or near their optimum in high productivity, a small difference in melt viscosity, as characterized by melt flow rate, will make a large difference in machine productivity of flaw-free parts. A higher melt flow rate with the same heat distortion temperature allows shorter injection cycles, which allows more parts to be produced per unit time. If the melt flow rate begins to drift lower, various defects in the part develop. These may, for example, range from weak weld lines or visible weld lines in a multi-gated mold, to visible surface defects, sink marks, edge defects, lighter parts, and finally to a short shot, as the given cycle causes insufficient material to flow into the mold.
If the melt flow rate should drift upward, other difficulties are seen due to overfilling of the mold. These may, for example, take the form of increased part weights which may cause several problems. The least of these problems is perhaps the added cost of the material; more severe are mold release problems, due to the thicker molded part being bigger than the mold (once pressure is released). Sometimes this sticking causes the cycle to be stopped while the part is forced out. Other times the part may be broken due to the force needed to expel it.
A "permanent" increase in melt flow rate, however, results in a shorter and more productive molding cycle, if heat distortion temperature (solidification temperature) is not lowered.
With the use of many plasticizers, such as mineral oil, the melt flow rate can be increased, but at the same time the heat distortion temperature is lowered. This results in parts that have an undesired lower use temperature and lengthens the molding cycle by requiring greater cooling time in the mold (the part must be solidified before it is removed from the mold or it will deform).